The ruddy turnstone is a small, stocky, brightly-patterned shorebird, named for its habit of turning over objects such as stones, shells and seaweed to uncover prey hidden beneath. During the breeding season, the adult’s striking ‘tortoiseshell’ plumage is reddish-brown with blackish-brown patches on the upperparts, and white on the underparts, with a black and white head, throat, neck and breast. A white rump, back, upper tail and wingbar are conspicuous in flight and contrast with the otherwise dark upperparts. This species has short, usually orangey-red legs, with unwebbed toes, and the beak is short, dark and wedge-shaped, with a slightly upturned tip. Non-breeding individuals are much duller and darker, with grayish-brown and blackish upperparts, and grey-brown on the head, neck and breast. Juveniles resemble the non-breeding adult, but are browner, with a paler head.

In breeding plumage, the female ruddy turnstone can usually be distinguished from the male by the more mottled and streaked crown, slightly duller upperparts, browner back to the neck, and flecking on the breast. The ruddy turnstone can be distinguished from the closely related black turnstone (Arenaria melanocephala) by its reddish plumage, more intricate patterning, and orange rather than dark legs. The calls of the ruddy turnstone include a short, staccato tuk-a-tuk and a long, rapid trill.